My opinion from personal, practical experience is that a newcomer to horses is best served to take several years of lessons on many different horses before contemplating owning one. It's fine to have that as a goal, and to do a LOT of window shopping, but get into a good lesson program and really learn how to ride and handle horses before buying. Depending on how much training the newcomer gets, a yearling may or may not be a good idea. If the trainer has several really young horses and is willing to work with the person on how to handle and train them, then it's probably going to work out ok, IF the person is patient and doesn't mind waiting 2,3 or even 4 years to ride that horse.
Horses are a lot like people in that they mature at different rates. Some are ready to go to the trainer and be lightly saddle broke at 2, others need to wait until they are 3 or even 4. Most of mine can get started by 2, but I have 1 who will wait at least another year, she's just not mentally ready to accept that amount of training yet. And even with waiting until she is 3 or 4, she may not end up being what I want under saddle, though genotypically and phenotypically she is exactly what I wanted. They have their own minds and their own personalities and you won't know for sure, until you have the horse going under saddle for at least a year, whether or not it will end up what you want.
Most newcomers are better off to buy a 'made' horse or a 'BTDT' (been there, done that) horse that's a little older and well trained. I love being able to find an old lesson horse that is being retired, they are generally well trained but have some issues. Usually they are sound enough for 1 person to ride, even frequently, but may not be able to handle 5 or more hour long lessons in a day. They may be a little hard mouthed, but that can be fairly easily remedied (usually). Sometimes they have arthritis and need some form of maintenance. But they will teach you to ride and keep you safe while you explore more things than you get in weekly lessons. Sometimes they're a little grumpy and not your favorite lesson mount, but with one owner and not a bunch of different riders, they will soften and become sweet. A mid-teens horse can be kept sound and ridable, many times long past the time you need such a horse. They're worth their weight in gold.
As for the personality changes, the horse turning into the Devil Incarnate, there are a couple of possibilities. If it happens within days of buying the horse, it may have been drugged. If it's months, then it's usually a discipline issue. And sometimes, if it's an older horse, it can be pain. Fix the pain or fix the discipline, you fixed the horse's attitude. If it was drugged, that's a whole 'nuther kettle of fish, and why a newcomer should have a trainer shop for or with them.